The problem is that FDA bans not just false or misleading claims about an off-label use's safety and efficacy. That is, it's not just preventing snake oil salesmen from peddling quick fixes that don't work. The agency bans all promotion of off-label uses, even if those uses have been proven to be safe and effective in clinical trials. Even if those uses are considered to be the standard of care for a given ailment. And even if a physician could be liable for malpractice for not administering a drug off-label.

He also examines the ban in a First Amendment context,

Few would argue that false or misleading claims in a commercial context should be protected by the First Amendment. However, a few decades worth of now-well established case law concludes that government may not categorically bar truthful and non-misleading speech simply because its purpose is to promote a commercial transaction. Instead, government must have a substantial interest in regulating the speech in question, and the regulation must directly advance that governmental interest and be no more extensive than necessary to do so.